1. Field of Invention
The present invention relates to the reprocessing of hydrofluoric acid to the proper purity and weight percent for reuse in the semiconductor manufacturing industry.
2. Prior Art
Hydrofluoric acid (HF) may be used as an acid etchant for removing silicon dioxide or similarly composed substances from semiconductor devices in a bath during the manufacture of these devices. Typically, these semiconductor devices are placed in a bath containing HF to permit the acid to remove the silicon dioxide (SiO.sub.2). Once the etchant is spent, that is becomes essentially ineffective, the etchant acid must be replaced with clean (effective) hydrofluoric acid. Traditionally, the spent acid was drained from the bath and disposed of, then clean, new hydrofluoric acid (HF) was added to the bath. Since hydrofluoric acid is highly corrosive and highly toxic, considerable care must be taken in disposing of the spent acid.
Distillation of HF is one alternative for reprocessing the spent acid for reuse. The distillation of HF at atmospheric pressures requires the use of special equipment and procedures. Since hydrofluoric acid attacks glass and any silica-containing materials, the use of these materials for distillation purposes is prohibited. Distillation vessels and conduit lines fashioned from stainless or other alloy steels is likewise prohibited due to the nature of the semiconductor industry. Semiconductor grade acid must be essentially free of metal contaminants, e.g., no greater than 500 ppb of metal contaminants.
It is appreciated that what is needed is an effective method for recycling spent hydrofluoric acid while providing a reprocessed HF that meets semiconductor industry standards and without incurring prohibitive costs for reprocessing HF, thus making the process commercially feasible.
The present invention describes a novel two-step distillation process which produces hydrofluoric acid that possesses both the purity and the requisite dilution of hydrofluoric acid for the semiconductor industry, at a cost that is justifiable for these industries.